Upgrading from old site

Up until 6 months ago I was using ZC 1.3.9e and was satisfied with it. My friends kept telling to me I should switch sites and I switched to a 3dcart. I have had NOTHING but trouble with it. So I am looking at coming back. When I come back I would want to have the site updated to the newest version but I am not finding much on what problems I may run into. Has anyone done an upgrade from 1.3.8 to 1.5? I have several mods or I would just start fresh (and still may)
thank you!

A proper upgrade is essentially a rebuild of your site, but keeping the database and upgrading the database content to the new format.

So, plan to do that and you'll have the best success.

Yes, basically it is a rebuild whole webstore with existing database(you need to upgrade it).

There are currently two supported versions with 1.5.0 being the newest and greatest. However, you mention "several mods" which means you need to make sure that all are compatible with 1.5.0 prior to trying to use them.